| Black carps,grass carps,sliver carps and bighead carps were collectively as four major Chinese carps,belongs to the rivers and lakes semi-gratory fish,the main stream of the Yangtze River is an important breeding site for the four major Chines carp?Historical data on the investigation and research on the early resources of the four major Chinese carp are mainly concentrated in the middle and upper reaches of the Yangtze River,while there are few systematic reports in the lower reaches of the Yangtze River.Recent years,factors such as water conservancy projects,waterway transportation,and river and lake barriers have caused the resources of the four major Chines carp to increase from the middle reaches of the Yangtze River to the downstream.The upper end of the lower reaches of the Yangtze River is connected to Poyang Lake,The special river-lake complex ecosystem is conducive to domestic fish breeding in and out of the lake.Therefore,our research focuses on the investigation of the temporal and spatial pattern of the early resources of the four major Chinese carps in the lower reaches of the Yangtze River,and the in-depth analysis of the early growth status of dominant species of sliver carps.Providing reference materials also helps to explore the survival and supplementary mechanisms of the salamander population,and more effectively protects its germplasm resources.In present study,we investigated and analyzed the spatial and temporal pattern of the early fish resources of the four major Chinese carps at three river sections at Anqing,Nanjing,and Nantong in the lower reach of the Yangtze river from April to August 2019;The study focused on the annual changes of larval and juvenile fish in the Anqing section from 2016 to 2018,and analyze the hydrological and environmental factors that affect the abundance of the four major Chinese carps larval and juveniles by using GAM models;The microstructures of otolith in silver carp larvae were measured and analyzed,the age information was obtained and the incubation period was calculated,and the growth characteristics of otolith and fish boby were studied.The main research results are as follows:1. The current status of the early-life stage resources of the four major Chinese carps in the lower reaches of the Yangtze River is:the species composition of the four major fish eggs and fries in the lower reaches of the Yangtze River is dominated by sliver carps,followed by bigheard carps and grass carps,with a small number of black carps.During the investigation,the eggs were only collected from the Anqing section.According to the velocity and development period,the egg spawning sites were estimated to be distributed in the section from the Mianchuan Beach to Balijiang section,60?140km upstream of Anqing sampling section.The total spawning size was 0.37×10~8ind.The average daily catch of four major Chinese carps in Anqing,Nanjing and Nantong was 9.38 ind./d,1.9 ind./d and 1.2 ind./d,respectively.Due to the close proximity to the spawning grounds,the emergence and outbreak time of fish eggs and fries in the Anqing section was earlier than Nanjing and Nantong section,and two peaks occurred in mid-late may and mid-july.The analysis shows that the abundance of fish eggs and fries in Anqing section is significantly positively correlated with water temperature,runoff and net flow;the abundance of fry in Nanjing section is only positively correlated with the flow through net;There is no correlation between the abundance of domestic fish fry and water temperature and net water flow.The difference in the density of fish eggs and fries between three sampling sites in each section may be caused by different coastal habitats and water dynamics.The results of cluster analysis show that the lower reaches of the Yangtze river are similar to each other,but Anqing section in July was separated,indicating that there was a large-scale breeding activity of domestic fish in the upper reaches of the Anqing section in July.2. From 2016 to 2018,3178 larvaes and juveniles of four major Chinese carps were collected from Anqing section,and the percentage of silver carp was increasing,while grass carp was decreasing.It is estimated amount of 21.70×10~8?14.62×10~8?12.05×10~8larvaes and juveniles of four major Chinese carps dfifting through the Anqing section in 2016 to 2018,respectively,showing a decreasing trend year by year.The abundance of Chinese carps has been increased intermittently since mid-April,and its peaked in mid-July.The Kruskal-wails test shower that,there were significant differences for larvae and juveniles with the highest drift density in the north of river and the lowest in the middle of the river.According to the results of generalized additive model,the hydrological factors that have significant influence on the abundance of larvae are water temperature,transparency,daily increasing rate of water level and discharge(p<0.05),the orders of the contribution to abundance of larvae were in sequence of the daily increasing rate of water level,transparency,water temperature and the daily increasing rate of discharge.Among them,the abundance of larvae was negatively correlated with the transparency,and positively correlated with the water temperature(24-27.3?),daily increasing rate of water level and discharge.The accumulation of deviance explained by the optimized GAM to the abundance of larvae was 69.8%.,which revealed the influence of hydrological conditions on the abundance of Chinese carps resources in early stage was revealed to a high degree.3. The microotoliths of three randomly selected batches of 150 sliver carps larvaes were analyzed,among which 115 were identified with otoliths.The average age was8.79±2.24d,mainly distributed in 6?10d.The incubation period based on the sampling date and age of larvae was May 5 to July 17.Regression analysis showed that each batch of silver carps larvae of otolith radius,length had significant linear relationship with age,but the correlation coefficient between batches are relatively low,that the Yangtze river downstream impaction wild growth larger differences between individuals,between different batches with high water temperature in July silver carps fry growth rate faster than the other months;The average growth rates of microotolith and total length of silver carp in Anqing section were 7.35 m/d and 0.34 mm/d,respectively.Compared with the middle reaches of the Yangtze river,the higher growth rates indicated that the Anqing section of high-density zooplankton provided a good feeding and breeding place for early growth of silver carp. |
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